1. Field of the Invention
The present invention relates to a semiconductor memory device.
2. Description of the Related Art
Examples of a semiconductor memory device (also referred to as a memory device or a memory cell) include a dynamic random access memory (DRAM), which is a volatile memory. The DRAM is a semiconductor memory device which can store one-bit data using one transistor and one capacitor. An area per unit memory cell of the DRAM is small; the degree of integration thereof can be easily increased at the time of making a module; and manufacturing cost thereof is low.
When a transistor including an oxide semiconductor is used for a DRAM, charge which is held in a capacitor in the DRAM can be held for a long time owing to low off-state current characteristics of the oxide semiconductor, which enables an interval between refresh operations to be extended. As a result, it is possible to reduce power consumption (see Patent Document 1).
In addition, in order to increase the operation speed and storage capacity of a semiconductor memory device, an increase in the degree of integration by a microfabrication technology is required. However, as microfabrication of a semiconductor memory device advances, a transistor having a channel with shorter channel length and including various insulating films typified by a gate insulating layer which have smaller thickness is used for a semiconductor memory device. As a result, leakage current of the transistor is increased, so that power consumption is increased.
To reduce an area occupied by a semiconductor memory device, an area occupied by a cell can be reduced by devising a layout of a circuit (see Patent Document 2).